HP 6115A #1937A00855 Minor Repairs

[pigrew]

6115A, $37USD from evilBay, mid 80s vintage, in as-is condition. It had many of the reported by others on this forum: a buzzing film cap across the transformer, bad contacts setting the voltage, very dirty PCB, oil oozing out of the clear plastic cable tubes, broken binding parts, and a bad electrolytic capacitor. Even still, it mostly worked when delivered. The only "major" issue was that the crowbar OVP circuit activated itself at about 90V.

I wouldn't call my work a restoration, but more of a refurbishment. I want a functioning, low-noise, 100 V supply, but not necessarily a piece to install in the Louvre.

The repairs are as follows:

• Replacing buzzing C1 film capacitor. This seems to be a common fault. The capacitor still tested good. I replaced with a X2-rated 1uF 310VAC R463N410050N2M. Using X2 was not necessary, but is a good idea. The orientation of the capacitor doesn't electrically matter.
• I bumped one of the ceramic 50nF Z5U capacitors, and its leg shed a bit of insulator. I replaced it with a tiny blue X7R RDER72J473K3K1H03B. Maybe I should have just replaced all of them?
• I didn't like the looks of C1, C2, C3, and C12 electrolytic capacitors. They were in the segmented packages with three ground lugs. After removing them, one proved to be a resistor (probably causing the OVP defect). The replacements (100ZLH470MEFC16X31  and EEU-EB2C221) are about 1/4 the size of the stock components. C12 is rated at 160V, though the old part was 175V. This cap is normally configured to be across the terminals. The manual specifies 150VAs the PCB only routed one negative terminal of each capacitor, the replacement was pretty easy.
• To further the goal of replacing electrolytic, C13 across the front-panel output lugs was replaced with 250BXC4.7MEFC8X11.5.
• The three front-panel binding posts were cracked, and needed replacement. I grabbed some from the junk drawer. I used larger lugs that cover the markings on the panel, oops. Yes, the ground should be green, but at least it's not the same color as the negative post?
• The front-panel decade switches were disassembled, cleaned with ethanol, and then lubricated with G5 Deoxit spray.
• The outside panels were washed with dish-detergent to make them less sticky.
• Isopropanol was used to wipe the oil off the PVC tubes used for cable routing.

At this point, the source is in good shape, though there are a few other tweaks which may be good to do:

• Add a mains EMI filter? This is likely not needed, and I don't have a clear idea on how to test if one should be added. The easiest way would be to add the proper X and Y capacitors across the input transformer lugs, though this wouldn't include the series inductor of an off-the-shelf filter.
• The millivolt potentiometer is crusty and doesn't make good contact. I should order some of the fader deoxit, and spray it down....
• Find a screw to replace the missing one holding the back-panel on.
• Calibration? I don't really need a cal sticker. I think it's good enough as it is.

All in all, this is a nice addition to my lab, and will enable me to adjust the 100VDC ranges of my multimeters (by transferring a measurement from one meter to another). I'm not worrying about thermal EMF in this case since the total voltage is so high, so no CuTe unobtanium is needed in this unit.

(P.S.: This is mostly a repost from the "what did I just waste my money on today" thread.)

[tautech]

Nice work ! 
Yep those caps look foooked and needing replacement.

EMI filter for a linear supply is not really required as the transformer itself and following regulation circuitry will absorb lots of mains noise unless the mains takes a big hit from somewhere and then a filter mightn't save the instrument anyways.